When you draw the direction of current through the diodes in your bridge converters and centre tap transformers,
the current is shown as flowing into the negative terminal of the diodes. Isnt this impossible?

maybe "swaroop" has already been enlightened by going back to his books. but i do believe many are confused by this concept of "conventional current flow" & "electron flow" I would like to quote from a book from Texas Inst and later in its 5th edition edited by Don L. Cannon Ph. D. and i quote,

"The direction in w/c electrons can pass is opposite the direction in w/c the arrowhead points. Electrons flow within the diode from cathode (K) to anode (A).

You may wonder why the diode symbol arrow points opposite to the direction of electron flow. In circuits, we like to use a current flow opposite to electron flow. Electrons flow from more negative voltages to more positive voltages. Circuit descriptions use an imaginary current that flows from more positive voltages to more negative voltages. This imaginary current flow called conventional current. The diode symbol points in the direction of conventional current flow, since it is a circuit symbol. The conventional flow is the direction current would flow if the flowing particle were positively charged. Physically we see that current is due to negativelycharged electrons flowing in one direction. This is equivalent to positively charged particles flowing in the opposite direction. In physical description of devices , we tend to use the physically meaningful electron flow. In circuit description, the conventional current tends to make our circuit simpler to understand. "